1. Field of the Invention
The invention relates to phase change switches and other control elements or devices, and more particularly, to phase change switches or control devices having a dynamic range of impedance, and circuits and components employing such switches or control devices. More specifically, the invention relates to such switches which can be employed in circuits such as on frequency selective surface arrays, for controlling current flow throughout the array, through the use of the switches. By controlling such current flow, the properties of the frequency selective surface array can be actively controlled. In addition, the invention also relates to implementation of such switches and other control devices in circuits, and the circuits themselves, that use conductive structures and dielectrics to guide electromagnetic (EM) waves.
2. Background of the Invention
Mechanical on/off switches have been used in circuits designed to interact with electromagnetic waves, and in particular, circuits designed to handle guided electromagnetic (EM) waves. Another set of such applications includes two-dimensional periodic arrays of patch or aperture elements known as frequency selective surfaces (FSS), the capabilities of which have been extended by addition of active devices, such as switches, and which are generally known as active grid arrays.
The mechanical process in these on/off switches involves the physical motion of a conductor (the xe2x80x9cbridgexe2x80x9d) between two positions, i.e., one where the bridge touches another conductor and completes the direct current (DC) conducting path of the circuit (xe2x80x9cclosedxe2x80x9d) or moves close enough to it that the capacitive impedance is low enough to complete the path for alternating current (AC) flow, and the other where it has moved away from the contact (xe2x80x9copenxe2x80x9d) to break the DC conducting circuit path or to raise the capacitive impedance to block AC flow. Such mechanical switches have been made at micrometer size scale in so-called MEMSxe2x80x94Micro-Electro Mechanical Systems. MEMS switch technology to date has shown poor lifetimes and packaging costs.
A key goal in the use of MEMS switches with guided EM waves in the so-called radio frequency (RF) bands is to provide controllable phase delays in a circuit. This is done by using a set of switches to introduce combinations of fixed length phase delay branches into a circuit path. The degree of phase delay control is related to how many separate branches (and switches to control them) are added to the circuit. The switching in or out of a given fixed delay branch provides a step change in the net circuit phase delay. In this approach, if finer steps are desired to cover the same range of total phase delay, then more branches and switches are required.
Alternatively, transistor and transistor-like semiconductor switching devices have been used in circuits designed to interact with electromagnetic waves and in particular, in circuits and components thereof that guide EM waves. Such devices which include PIN diodes and field effect transistors (FETs) form the basis of a collection of solid-state circuits operating on guided EM waves of up to gigahertz (e.g., GHz, 1 GHz≅109 Hz) for use in microwave and communication systems. However, for the specific applications herein, the semiconductor switching devices typically have shortcomings in several areas, i.e., GHz and above. Such shortcomings may include high switching power required or high insertion losses.
In the field of semiconductor memory devices, it has been proposed to use a reversible structural phase change (from amorphous to crystalline phase) thin-film chalcogenide alloy material as a data storage mechanism and memory applications. A small volume of alloy in each memory cell acts as a fast programmable resistor, switching between high and low resistance states. The phase state of the alloy material is switched by application of a current pulse, and switching times are in the nanosecond range. The cell is bi-stable, i.e., it remains (with no application of signal or energy required) in the last state into which it was switched until the next current pulse of sufficient magnitude is applied.
In accordance with one aspect of the invention there is provided a switch or control element or device for use in circuits and components that interact with electromagnetic radiation, and more specifically, in circuits or components that guide EM waves. The switch or control element or device includes a substrate for supporting components of the switch. A first conductive element is on the substrate for connection to a first component of the circuit or component (hereafter collectively xe2x80x9ccircuitxe2x80x9d), and a second conductive element is also provided on the substrate for connection to a second component of the circuit. Such switches and circuits involve implementations to guide EM waves in circuits such as parallel wire transmission lines, coaxial cables, waveguides, coplanar waveguides, striplines and microstriplines. Use of such switch devices allows control of energy flow through the circuits with functional properties such as fast switching times, e.g, about 10 nanoseconds to about 1 microsecond; low insertion loss, e.g., about 1 dB or less; high isolation, e.g., about 20 dB or higher; long lifetime, e.g., at least about 1013 cycles; and low cost. Addressing of the control devices either electrically or optically allows flexibility in how the devices are used.
A circuit for guiding electromagnetic waves includes a substrate for supporting components of the circuit for guiding the electromagnetic waves and at least one control device. The control device includes at least one conductive element on the substrate for connection to at least one component of the circuit. A second conductive element is provided on the substrate for connection to at least one second component of the circuit and the control device is made up of a variable impedance switching material on the substrate. The switching material connects the at least one first conductive element to the at least one second conductive element. The switching material is made up of a compound which exhibits a bi-stable phase behavior, and is variably switchable to an impedance between the first impedance state value and up to a second impedance state value by application of energy thereto. As a result, the switching affects the amplitude and/or phase delay of electromagnetic waves through the circuit as a result of a change in the impedance value of the compound. Similarly, the path of the guided EM waves can also be affected and/or controlled.
In more specific aspects, the first and second impedance state values are such that at one value the control device is conductive, and at the other value the control device is less conductive or non-conductive. Preferably an energy source is connected to the control device for causing the change in impedance value. The energy source can be an electrical energy source with leads connected to the switch. Alternatively, the energy source could be a light source which is a laser positioned to direct a laser beam to the switch or control device to cause the change in impedance value. In a more specific aspect, fiber optics or an optical waveguide is associated with the laser and the switch to direct the laser light to the switch.
The circuit and components can be a circuit or component employing or made up as parallel wire transmission lines, coaxial cables, waveguides, coplanar waveguides, striplines, or microstriplines. The material making up the switch or control device is preferably a chalcogenide alloy, and more preferably at least one of Ge22Sb22Te56, and AgInSbTe.
In a more preferred aspect, in some applications, the compounds for the control device are used in a range of stable intermediate stage set on a submicron scale or mixtures of amorphous and crystalline phases, but which exhibit (average) intermediate properties under larger scale measurement or functional conditions.
In an alternative aspect, the invention is directed to a control device for use in circuits which guide electromagnetic waves. The control device is made up as previously described herein.